A terminal in a radio system comprises a mobile equipment as an operational unit and a SIM module as a user-specific module, of which the mobile equipment comprises parts required for handling, transmitting and receiving a signal. The SIM module, in turn, comprises a processor and memory resources for processing and storing user data and user interface data.
The mobile equipment and the SIM module can de made mutually compatible such that a particular, single mobile equipment only works with a particular, single SIM module. So, when the mobile equipment is switched on, it locks by means of software to a correct SIM module attached to said mobile equipment. The operator of the user interface for making and receiving calls is determined in the SIM module. This is the case, for instance, when, in connection with the user interface purchase, the operator wishes to provide the user with a mobile equipment that is only usable for calls from the user interface in question. Because the user interface data is stored in the SIM module, it is possible to prevent the mobile equipment from being used with another SIM module, for instance, in another operator's network, by preventing the operation of the mobile equipment if an inappropriate SIM module is attached to the mobile equipment. Therefore, the mobile equipment comprises a program, by which it checks in connection with start-up that the SIM module attached to the terminal equipment is the appropriate one.
However, current SIM module check programs have a drawback that the SIM module check program in the mobile equipment can be altered too easily in such a manner that the mobile equipment is made operable also with other SIM modules than the one intended.